Hydrodynamic gas bearing

ABSTRACT

A hydrodynamic gas bearing includes a cantilevered shaft having a free end, a fixed end, and a communicating hole for communicating with a passage formed at an end surface of the free end of the shaft. A cover member is rotatably arranged around the shaft to cover a periphery of the shaft. One of an inner surface of the cover member which faces the fixed end of the shaft and an outer surface of the fixed end of the shaft has herringbone-configured hydrodynamic pressure generating grooves for preventing gas from being fed from a fixed end side to a free end side of the shaft. One of the inner surface of the cover member which faces the free end of the shaft and an outer surface of the free end of the shaft has helical-configured hydrodynamic pressure generating grooves for feeding gas from the free end side to the fixed end side of the shaft.

BACKGROUND OF THE INVENTION

The present invention relates to a hydrodynamic gas bearing, providedfor the main shaft of a polygon mirror scanner or the like, for floatinga sleeve relative to a shaft to constitute a radial bearing and a thrustbearing by pressure generated in dynamic pressure generating groovesprovided in the radial bearing, and more particularly to a hydrodynamicgas bearing which generates no whirls or self-excited vibrations andprovides stable rotation.

FIGS. 7 and 8 show an example of a conventional hydrodynamic gas bearingapplied to a polygon mirror scanner.

Referring to FIG. 7 which is a sectional view showing the conventionalhydrodynamic gas bearing, the lower end of a shaft 12 is fixed to thecenter portion of a main body 11 of the bearing. There are provided inthe periphery of the shaft 12 a pair of dynamic pressure generatingherringbone-configured hydrodynamic pressure generating grooves 12A. Aperipheral groove 12B is provided in the region, between theherringbone-configured hydrodynamic pressure generating grooves 12A, inwhich a high pressure is generated. There are provided in the shaft 12 acommunicating hole 12C and a throttling hole 12D for feeding pressurizedgas under pressure from the peripheral groove 12B to the end surface ofthe free end of the shaft 12. There is provided a rotatable lid 17positioned on the free end of the shaft 12. A flange 13B for mounting apolygon mirror 19 to a sleeve 13 is provided in the vicinity of theupper end of the sleeve 13. The lid 17 serves as a means for fixing thepolygon mirror 19 to the main body 11 by a bolt. A rotor magnet 15 ismounted on the sleeve 13 and a stator 14 is fixed to the main body 11.The stator 14 and the rotor magnet 15 constitute a motor 16. Aprotective dust cover 20 having a glass 20A is fixed to the main body11.

The operation of the hydrodynamic gas bearing of the above-describedconstruction is described below with reference to FIGS. 7 and 8.Referring to FIG. 7, upon energizing of the stator 14 of the motor 16,the rotor magnet 15 is rotated. While the lid 17 and the polygon mirror19 rotate together with the sleeve 13 at a speed as high as, forexample, 30,000 r.p.m., laser beams are incident on the polygon mirror19 as shown by the arrow (D) and reflected thereby. During rotation ofthe sleeve 13, the sleeve 13 rotates at high speed with a gas pressureincreasing between the herringbone-configured hydrodynamic pressuregenerating groove 12A and the sleeve 13 due to a pumping action of theherringbone-configured hydrodynamic pressure generating grooves 12A. Apart of the gas thus pressurized is introduced from the peripheralgroove 12B to the free end of the shaft 12 via the communicating hole 12and is then blown out toward the lid 17 through the throttling hole 12Dconsisting of a slit. Thus, a force for floating the sleeve 13 relativeto the shaft 12 in the direction shown by the arrow (A) is generated.The weight of the sleeve 13, the mirror 19, and the lid 17 and the forceof the magnet 15 for attracting the stator 14 act in the direction shownby the arrow (B). As a result, the forces acting in the direction shownby the arrows (A) and (B) balance each other. Thus, the sleeve 13 floatsin an amount shown by the arrow (C). FIG. 8 shows the distribution ofthe gas pressure in the radial direction generated by theherringbone-configured hydrodynamic pressure generating grooves 12A andthat of the pressure in the thrust direction generated by the blow-outof the gas from the communicating hole 12C. The protective cover 20 andthe glass 20A prevent dust from penetrating into the bearing and dewfrom being formed on the polygon mirror 19.

However, the above construction has the following disadvantages: The gaspressure in the vicinity of the fixed end of the shaft 12 does notincrease sufficiently and thus the fixed end of the shaft 12 seizes anda whirl or a self-excited vibration is generated, which may lead to anunstable rotation of the bearing. Particularly, the communication of theherringbone-configured hydrodynamic pressure generating grooves 12A withthe peripheral groove 12B or the communicating hole 12C reduces pressureof the gas and generates the whirl to a great extent.

SUMMARY OF THE INVENTION

It is therefore an essential object of the present invention to providea hydrodynamic gas bearing capable of preventing whirls or self-excitedvibrations from occurring, such that it will provide for stablerotation.

In accomplishing these and other objects, according to one aspect of thepresent invention, there is provided a hydrodynamic gas bearingcomprising: a cantilevered shaft having a free end, a fixed end, and acommunicating hole for communicating with a passage formed at an endsurface of the free end of the shaft; and a cover member rotatablyarranged around the shaft to cover a periphery of the shaft. One of aninner surface of the cover member which faces the fixed end of the shaftand an outer surface of the fixed end of the shaft hasherringbone-configured hydrodynamic pressure generating grooves forpreventing gas from being fed from a fixed end side to a free end sideof the shaft. One of the inner surface of the cover member which facesthe free end of the shaft and an outer surface of the free end of theshaft has helical-configured hydrodynamic pressure generating groovesfor feeding gas from the free end side to the fixed end side of theshaft.

BRIEF DESCRIPTION OF THE INVENTION

These and other objects and features of the present invention willbecome apparent from the following description taken in conjunction withpreferred embodiments thereof with reference to the accompanyingdrawings, throughout which like parts are designated by like referencenumerals, and in which:

FIG. 1 is a sectional view showing a hydrodynamic gas bearing accordingto an embodiment of the present invention;

FIG. 2 is a descriptive view showing a pressure generated in the bearingof FIG. 1;

FIG. 3 is a bottom view of the lid of the bearing;

FIG. 4 is a graph showing pressure changes in the thrust and radialdirections of the bearing;

FIG. 5 is a sectional view showing a hydrodynamic gas bearing accordingto another embodiment of the present invention;

FIG. 6 is a descriptive view showing a pressure generated in the bearingof FIG. 5;

FIG. 7 is a sectional view showing a conventional hydrodynamic gasbearing; and

FIG. 8 is a descriptive view showing a pressure generated in the bearingof FIG. 7.

DETAILED DESCRIPTION OF THE INVENTION

Before the description of the present invention proceeds, it is to benoted that like parts are designated by like reference numeralsthroughout the accompanying drawings.

Referring to FIGS. 1 and 2, a hydrodynamic gas bearing according to anembodiment of the present invention is described below. In thisembodiment, the bearing is applied to a polygon scanner. Referring toFIG. 1, the lower end portion of a shaft 2 is fixed to the centerportion of the main body 1 of the bearing. There are provided in theperiphery of the shaft 2 a helical-configured hydrodynamic pressuregenerating grooves (referred to as helical groove hereinafter) 2A forfeeding gas to the fixed end side of the shaft 2 and aherringbone-configured hydrodynamic pressure generating grooves(referred to as herringbone grooves hereinafter) 2D having anapproximately symmetrical configuration as shown in FIG. 1. Morespecifically, the grooves 2A are positioned on the outer surface of thefree end of the shaft 2 and the groove 2D are positioned on the outersurface of the fixed end of the shaft 2. There are provided, between thehelical and herringbone grooves 2A and 2D, a peripheral groove 2B havingopposing holes 2F opening into a communicating hole (or passage) 2C. Thecommunicating hole 2C is so formed that the opposing holes 2F formed inthe peripheral groove 2B and a throttling hole 2E formed in the endsurface of the free end of the shaft 2 are communicated with each otherthrough the communicating hole 2C, so that pressurized gas is fed fromthe opposing holes 2F formed in the peripheral groove 2B to thethrottling hole 2E formed in the end surface of the free end of theshaft 2 through the communicating hole 2C. The throttling hole 2E is soformed that a cylinder tube 2G having a narrow passage is closely fittedin a large hole formed at the upper end portion of the communicatinghole 2C. A cover member is comprised of a rotatable sleeve 3 and a lid 7provided on the free end side of the shaft 2. The sleeve 3 is providedaround the shaft 2 to rotate around the shaft 2. The lid 7 has a throughhole 7A and a groove 7B for communicating with the through hole 7A atthe inner surface of the lid 7 which faces the end surface of the freeend of the shaft 2 as shown in FIG. 3. When the sleeve 3 is rotating atthe high speed and large vibration are applied to the bearing in thethrust direction, the lid 7 and the sleeve are vibrated in the thrustdirection. However, even if the end surface of the free end of the shaft2 contacts the inner surface of the lid 7, the through hole 7A of thelid 7 is not closed by the end surface of the free end of the shaft 2because the groove 7B communicating with the through hole 7A cannot beclosed by the end surface of the shaft 2. As a result, as shown in FIG.4, the gas pressure change in the thrust direction is relatively smalland the gas pressure in the radial direction is hardly changed. Thus,the sleeve 3 stably rotates without radial pressure change even whenthrust vibrations are applied to the bearing. The sleeve 3 has at itsmiddle portion a flange 3B for fixing a polygon mirror 9 to the mainbody 1 by holding the mirror 9 between the flange 3B and the lid 7. Thelid 7 serves as a means for fixing the polygon mirror 9 to the main body1 by a bolt. A rotor magnet 5 is mounted on the sleeve 3. A stator 4corresponding to the magnet 5 is fixed to the main body 1. The stator 4and the rotor magnet 5 constitute a motor 6. A protective dust cover 10having a glass 10A is attached to the main body 1.

The operation of the hydrodynamic gas bearing of the above-describedconstruction is described below with reference to FIGS. 1 and 2.Referring to FIG. 1, upon energizing of the stator 4 of the motor 6, themagnet 5 is rotated. While the lid 7 and the polygon mirror 9 rotatetogether with the sleeve 3 at a speed as high as, for example, 30,000r.p.m., laser beams are incident on the polygon mirror 9 as shown by thearrow (D) in FIG. 1 and reflected thereby. During the rotation of thelid 7, the polygon mirror 9, and the sleeve 3, the sleeve 3 rotates atthe high speed with a gas pressure increasing between the shaft 2 andthe sleeve 3 due to a pumping action caused by the helical groove. 2Aand the herringbone grooves 2D. As a result, the sleeve 3 rotates aroundthe shaft 2 without contact of the shaft 2 with the sleeve 3. Thepressurized gas fed from the free end side of the shaft 2 to the fixedend side thereof by means of the helical groove 2A is introduced to thefree end of the shaft 2 from the opposing holes 2F of the peripheralgroove 2B through the communicating hole 2C, and is then blown outtoward the lid 7 through the throttling hole 2E. As a result, thepressure between the end surface of the free end of the shaft 2 and thesleeve 3 is increased and a part of the pressurized air escapes throughthe throttling passage 7A so that the pressure is decreased to maintaina constant pressure value. Thus, an approximately constant force isgenerated for floating the sleeve 3 in the direction shown by the arrow(A) in FIG. 1. The weight of the rotary body, that is, the sleeve 3, thepolygon mirror 9, and the lid 7 and the force of the magnet 5 forattracting the stator 4 act in the direction shown by the arrow (B) inFIG. 1. The forces acting in the directions shown by the arrows (A) and(B) balance each other. Thus, the sleeve 3 floats in an amount shown bythe arrow (C) in FIG. 1. FIG. 2 shows the distribution of the gaspressure in the radial direction generated by the helical grooves 2A andthe herringbone grooves 2D and that of the gas pressure in the thrustdirection generated by the blow-out from the throttling hole 2E. In FIG.2, the distribution is shown by solid lines and the distribution of thegas pressure in the radial direction generated by the two helicalgrooves 12A in the conventional bearing shown in FIGS. 7 and 8 is shownby the dotted line. As a result, the gas pressure in the radialdirection generated by the helical grooves 2A and the herringbone groove2D is sufficiently higher than that in the conventional bearing,especially in the vicinity of the fixed end of the shaft 2. Theprotective cover 10 and the glass 10A prevent dust from penetrating intothe bearing and dew from being formed on the polygon mirror 9.

As apparent from the forgoing description, since gas pressure isincreased greatly by the action of the helical grooves 2A and theherringbone grooves 2D, whirls and self-excited vibrations do not occurduring rotation of the sleeve 3, the mirror 9, and the lid 7 and, thus,the above members 3, 9, and 7 rotate stably. Particularly, since eachmaximum pressure of the gas pressure distribution is sufficiently high,the bearing operates reliably.

Even when the helical grooves 2A and/or the herringbone grooves 2D areformed in the inner periphery of the sleeve 3, effects similar to theabove can be obtained and the gas pressure increases in a similarmanner. Additionally, the groove 7B can be formed on the end surface ofthe free end of the shaft 2 to communicate with the through hole 7A ofthe lid 7.

Referring to FIGS. 5 and 6, a hydrodynamic gas bearing according toanother embodiment of the present invention is described below. In thisembodiment, the bearing is also applied to a polygon scanner. Referringto FIG. 6, the lower end portion of the shaft 2 is fixed to the centerportion of the main body 1 of the bearing. There are provided in theperiphery of the shaft 2 first and second herringbone grooves (referredto as herringbone grooves hereinafter) 2K and 2D. The first herringbonegrooves 2K are positioned on the periphery of the free end of the shaft2 and have an asymmetrical configuration so that the grooves 2K feed gasfrom the free end side to the fixed end side of the shaft 2. The secondherringbone grooves 2D are positioned on the periphery of the fixed endof the shaft 2 and have an approximately symmetrical configuration sothat the grooves 2D prevents gas from being fed from the fixed end sideto the free end side of the shaft 2. In this embodiment, theabove-described effect can be obtained.

As apparent from the foregoing descriptions of the embodiments of thepresent invention, high pressure is generated by the pumping operationof the helical and herringbone grooves 2A and 2D or the herringbonegrooves 2K and 2D. Therefore, whirls and self-excited vibrations do notoccur and the sleeve rotates stably.

Although the present invention has been fully described in connectionwith the preferred embodiments thereof with reference to theaccompanying drawings, it is to be noted that various changes andmodifications are apparent to those skilled in the art. Such changes andmodifications are to be understood as included within the scope of thepresent invention as defined by the appended claims unless they departtherefrom.

What is claimed is:
 1. A hydrodynamic gas bearing comprising:acantilevered shaft having a free end, a fixed end, a free end outerperipheral surface, a fixed end outer peripheral surface, and acommunicating passage opening through an end surface of said free end ofsaid shaft; a cover member rotatably arranged around said shaft to covera periphery of said shaft; wherein a first herringbone-configuredhydrodynamic pressure generating groove is formed in one of said fixedend outer peripheral surface and a portion of an inner surface of saidcover member facing said fixed end outer peripheral surface for feedinggas between said cover member and said free end outer peripheral surfaceof said shaft toward said fixed end of said shaft; wherein a secondherringbone-configured hydrodynamic pressure generating groove is formedin one of said free end outer peripheral surface of said shaft and aportion of said inner surface of said cover member facing said free endouter peripheral surface for preventing gas from being fed from saidfree end toward said fixed end of said shaft; and wherein a through holeis formed in a portion of said cover member facing said end surface ofsaid free end of said shaft, and a communication groove is formed in oneof said end surface of said free end of said shaft and said portion ofsaid inner surface of said cover member facing said end surface of saidfree end of said shaft, such that said communication groove communicateswith said through hole.
 2. The hydrodynamic gas bearing as claimed inclaim 1, whereinsaid second herringbone-configured hydrodynamic pressuregenerating groove has an approximately symmetrical configuration alongan axial direction of said shaft.
 3. The hydrodynamic gas bearing asclaimed in claim 1, whereinsaid first herringbone-configuredhydrodynamic pressure generating groove has an asymmetricalconfiguration along an axial direction of said shaft.
 4. Thehydrodynamic gas bearing as claimed in claim 1, whereina peripheralgroove is formed in said shaft at a portion thereof between said firstand second herringbone-configured hydrodynamic pressure generatinggrooves.
 5. The hydrodynamic gas bearing as claimed in claim 1,whereinsaid communication groove is formed so as to define a means formaintaining communication between said through hole and an interior ofsaid cover member even if said end surface of said free end of saidshaft is in contact with said portion of said cover member facing saidend surface of said free end of said shaft.
 6. The hydrodynamic gasbearing as claimed in claim 5, whereinsaid cover member includes, atsaid portion of said inner surface thereof facing said end surface ofsaid free end of said shaft, an inwardly protruding portion protrudinginwardly toward said shaft and having a radially outwardly facing sidesurface; said through hole opens through said inwardly protrudingportion; and said communication groove is formed in said inwardlyprotruding portion and opens inwardly toward said end surface of saidfree end of said shaft and through said radially outwardly facing sidesurface of said inwardly protruding portion.
 7. The hydrodynamic gasbearing as claimed in claim 1, whereinsaid cover member includes, atsaid portion of said inner surface thereof facing said end surface ofsaid free end of said shaft, an inwardly protruding portion protrudinginwardly toward said shaft and having a radially outwardly facing sidesurface; said through hole opens through said inwardly protrudingportion; and said communication groove is formed in said inwardlyprotruding portion and opens inwardly toward said end surface of saidfree end of said shaft and through said radially outwardly facing sidesurface of said inwardly protruding portion.